It says in SA Siman 638 that one may make a tanai (condition) on the "noi sukkah" (sukkah decorations) in order he should be able to use it during the chag and that it won't become muktzeh l'mitzvaso (set aside for the mitzva). In theory could one do this with the scach as well (so that it won't become muktzeh and the person will be able to be move it in a regular manner). If not, what is the reason why?

1 Answer
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All the above [leniencies] apply [only] to the decoration of the sukkah, but not to the branches used for the actual s'chach. A stipulation to make use of them is never effective, even if the sukkah collapses during the eight days of the festival, for the prohibition against [using] this is of Scriptural origin, as stated above.

[Moreover, this stringency applies] even to the wall of the sukkah, although their [use is] prohibited only by Rabbinic decree. Since they were designated to be used for the mitzvah of the sukkah, the holiness of the sukkah rests upon them and a stipulation to make use of them is of no consequence. [This applies] even if the sukkah falls during the festival.

It continues with an explanation, plus some extra details for other situations (in particular for the walls), but it's too much to type out.